Quantum Mechanics Introduces the ‘Dropleton’

Today physicists in Germany and the USA announced a new discover in quantum mechanics, introduced as the “dropleton,” or “quantum droplet.” The dropleton is a new quasi-particle that succinctly resolves an oddity in quantum mechanics research.

How does one discover a particle that is unimaginably small? It turns out the discovery of the dropleton was accessory to another research project that researchers from JILA and the University of Colorado Boulder were conducting on biexcitons.

As the name implies, biexcitons are formed from two excitons—or quasi-particles that are formed from the union of an electron and a hole. To study biexcitons scientists employed techniques from the field of ultrafast optics. Ultrafast optics is the study of precisely emitting incredibly short pulses of light (measured in fetoseconds, or 10^-15 seconds). These techniques are used to analyze how matter interacts with light during the briefest of time intervals— a process that is somewhat similar to looking at a series of rapidly-snapped pictures of a moving object (think of a stroboscope). When a laser was beamed at a sample of gallium arsenide (GaAs) and set to emit pulses of light over 100 million times per second, researchers expected the energy of the system increase as a result creating more electrons and holes. Instead what they saw was a perplexing decrease in energy.

Puzzled, the American researchers consulted their German collaborators from the Philipps-University Marburg. The team from Germany proposed that the Americans had unwittingly created a structure in which there were multiple electrons and multiple holes. This structure would later be referred to as a dropleton.

Visualizing such a structure involves combining both the traditional model of the atom (a positive nucleus with negative electrons orbiting around it) and the antiquated “plum pudding” model of the atom (in which an atom is a large sphere of positive charge with deposits of negative charge embedded throughout). Except instead of their being the familiar protons, neutrons, and electrons, the dropleton is composed of a limited number of holes and electrons. These holes and electrons are not paired off as they are in biexcitons, but instead interact equally throughout. This configuration is why the researchers did not see the increase in energy that they had originally anticipated.

As with most of quantum mechanics, introducing the concept of the dropleton is more a matter of defining probabilities than a concrete structure. A good place to start is at the top of the tall dropleton structure where one is most likely to find an electron. The holes are trickier to place. Most likely one would find them directly adjacent to the apical electron (though above, below, or to the side is anyone’s guess). If not there one might find a hole in the first ring, and if not there perhaps in the third ring. Holes are occasionally found in the second ring, but virtually never in the gaps between the rings. In addition it is important to note that the more electrons and holes are integrated into a single dropleton, the more rings of probability will be formed.

Overall this dropleton probability model provides an elegant answer researcher’s puzzling experimental results. The findings from this research were submitted to the prestigious academic journal Nature. Though it is still early to say, the introduction of the dropleton into the lexicon of quantum mechanics may yet answer other questions in the field of theoretical applied physics.